Means for restoring a tooth that is damaged by, for example, decaying, generally, varies depending upon the degree of damage to the tooth. In the initial to middle stage of decaying, for example, a cavity formed in the tooth is relatively small. In this case, a direct restoration is employed, in many cases, to directly fill the cavity with a curable restorative from such standpoints that the restoring work can be done easily and quickly, and that the restored portion exhibits excellent aesthetic appearance. If the degree of damage is large and a relatively large cavity is formed in the tooth, an indirect restoration is employed, in many cases, to join a prosthetic material (comprising, for example, a metal, ceramics or resin) to the portion of the tooth that is to be restored, the prosthetic material having been prepared in advance outside of the oral cavity.
Here, the restorative such as a composite resin or a prosthetic material has no adhesiveness to the tooth. To adhere it to the tooth, therefore, use is made of a dental adhesive comprising a curable composition that contains a polymerizable monomer component and a polymerization initiator component. In such a dental adhesive, a (meth)acrylate type monomer is used as a chief component of the polymerizable monomer, a photopolymerization initiator is used as a polymerization initiator, and a cured body is, usually, formed by the radical polymerization upon being irradiated with light.
However, there have also been known dental adhesives comprising curable compositions containing a chemical polymerization initiator (see, for example, patent documents 1 to 8). For example, even if it is attempted to directly restore a greatly damaged tooth with its crown destroyed by using a curable restorative, i.e., even if the portion to be restored is irradiated with intense light, light fails to reach the bottom portions thereof to a sufficient degree. In this case, therefore, there is used an adhesive that contains a chemical polymerization initiator as described above. Further, if the indirect restoration is carried out by using a prosthetic material that does not permit the transmission of light, such as a metallic prosthetic material, the adhesive cannot be sufficiently cured by photopolymerization, either. In this case, too, therefore, there is used an adhesive containing a chemical polymerization type polymerization initiator. Namely, the chemical polymerization initiator contains a radical-generating species and a reactive species, and generates radicals which serve as polymerization-initiating species upon contacting these two components together. Therefore, the adhesive containing the chemical polymerization initiator undergoes the polymerization to form a cured body without the need of being irradiated with light.
Further, there have been developed adhesives containing a photopolymerization initiator as well as an acidic group-containing polymerizable monomer having adhesiveness to teeth in order to obtain a higher adhering strength (see, for example, patent documents 1 and 2). That is, acidic groups such as phosphoric acid group and carboxylic acid group have a high affinity to the teeth (hydroxyapatite or collagen). By using a polymerizable monomer having such an acidic group, a cured product can be formed that is highly adhesive to the teeth. Upon making present water together therewith, further, the adhesive containing the acidic group-containing polymerizable monomer exhibits both the demineralizing function (etching capability) and the penetrating function to the teeth owing to the action of the acidic group. Upon being applied only once, the adhesive adheres highly to the teeth even without using primer and can, therefore, be used as a one-step type adhesive offering excellent operability (see, for example, patent documents 4, 9 and 10).
There have, further, been proposed dental adhesives blended with a polyvalent metal ion-eluting filler in addition to being blended with the acidic group-containing polymerizable monomer and water (see, for example, patent documents 4 to 12). Here, the polyvalent metal ion-eluting filler stands for a filler that elutes out ions of a polyvalent metal such as an alkaline earth metal or aluminum in an acidic solution such as a fluoroaluminosilicate glass. With the above adhesives, polyvalent metal ions eluted out from the polyvalent metal ion-eluting filler at the time of curing ionically bond to the acidic groups of the acidic group-containing polymerizable monomer to form ionic crosslinking. Namely, the polymerizable monomer is polymerized while the ionic crosslinking is being formed contributing to greatly increasing the strength of the cured body that is obtained and, as a result, to further increasing the strength of adhesion to the teeth.
With the above adhesives, however, ionic crosslinking occurs excessively during the storage to form a gel. Therefore, the polyvalent metal ion-eluting filler is stored in a package separate from a package that contains the acidic group-containing polymerizable monomer and water. Just before the use, the components contained in the packages are mixed together and used.
There has, further, been proposed a dental adhesive using, as the acidic group-containing polymerizable monomer, a polymerizable monomer that contains a phosphoric acid group, the polymerizable monomer being blended in the form of a salt such as calcium salt. It has been reported that this adhesive exhibits excellent strength of adhesion to the teeth (patent document 3).
The above conventional means for improving adhesion are chiefly applied to the dental adhesives of the photopolymerization type containing photopolymerization initiators. If these means are applied to the adhesives of the chemical polymerization type that contain chemical polymerization initiators and that undergo polymerization and curing without being irradiated with light, however, satisfactory adhesion is not obtained. That is, according to the study conducted by the present inventors, the chemical polymerization type adhesive containing, for example, the acidic group-containing polymerizable monomer and the polyvalent metal ion-eluting filler exhibits a considerably large strength of adhesion in the initial stage of polymerization and curing. After the passage of a predetermined period of time in a severe environment in an oral cavity, however, the strength of adhesion decreases sharply and, therefore, it becomes very highly probable that the composite resin or the prosthetic material peels off.
Patent document 1: JP-A-52-113089
Patent document 2: JP-A-58-21687
Patent document 3: JP-A-53-113843
Patent document 4: JP-A-2001-72523
Patent document 5: JP-A-2006-299201
Patent document 6: Leaflet of International Publication WO03/027153
Patent document 7: JP-A-2007-91607
Patent document 8: JP-A-2001-122718
Patent document 9: JP-A-9-309811
Patent document 10: JP-A-2004-529946
Patent document 11: JP-A-10-236912
Patent document 12: JP-A-2000-86421